As described by F. Capasso et al. in Solid State Communications, Vol. 102, No. 2-3, pp. 231-236 (1997) and by J. Faist et al. in Science, Vol. 264, pp. 553-556 (1994), which are incorporated herein by reference, a QC laser is based on intersubband transitions between excited states of coupled quantum wells and on resonant tunneling as the pumping mechanism. Unlike all other semiconductor lasers (e.g., diode lasers), the wavelength of the lasing emission of a QC laser is essentially determined by quantum confinement; i.e., by the thickness of the layers of the active region rather than by the bandgap of the active region material. As such it can be tailored over a very wide range using the same semiconductor material. For example, QC lasers with AlInAs/GaInAs active regions have operated at mid-infrared wavelengths in the 3 to 13 .mu.m range. In diode lasers, in contrast, the bandgap of the active region is the main factor in determining the lasing wavelength. Thus, to obtain lasing operation at comparable infrared wavelengths the prior art has largely resorted to the more temperature sensitive and more difficult-to-process lead salt materials system.
More specifically, diode lasers, including quantum well lasers, rely on transitions between energy bands in which conduction band electrons and valence band holes, injected into the active region through a forward-biased p-n junction, radiatively recombine across the bandgap. Thus, as noted above, the bandgap essentially determines the lasing wavelength. In contrast, the QC laser relies on only one type of carrier; i.e., it is a unipolar semiconductor laser in which electronic transitions between conduction band states arise from size quantization in the active region heterostructure.
Although most of the literature has focused on QC optical (i.e., photon) sources for operation as coherent, stimulated emission sources (e.g., lasers), these sources also find application as incoherent, spontaneous emission devices (akin to LEDs although carrier injection across a p-n junction is not involved). Such QC sources, especially lasers, have a variety of potential uses; for example, in trace gas analysis, environmental monitoring, industrial process control, combustion diagnostics, and medical diagnostics.
In at least some of the potential uses (as well as in quantum optics studies) it would be desirable to have available a semiconductor optical source that emits light at more than one wavelength. For example, such a source would be extremely useful for those techniques, like differential absorption lidar (DIAL), where scattering has to be evaluated and compared at two different wavelengths. In addition, if the photons that are emitted at the two wavelengths are correlated, then such a source would make it possible to eliminate spontaneous emission noise in measurements that require beating or heterodyning of two laser emissions. This application discloses such an optical (photon) source.
Dual wavelength QC lasers have been described in the prior art. For example, copending patent Ser. No. 5,978,397 filed on Mar. 27, 1997 (Capasso et al. 43-74-7-11-8-12 entitled Article Comprising an Electric Field-Tunable Semiconductor Laser) and assigned to the assignee hereof, discloses, inter alia, a divided-electrode QC laser that can emit light at two wavelengths. Such a laser requires more complex circuitry, and the two wavelengths have relatively small separation. Another illustration, copending patent application Ser. No. 09/033,250 filed on Mar. 2, 1998 (Capasso et al. 48-80-12-15-12-17-1 entitled Article Comprising a Dual-Wavelength Quantum Cascade Photon Source) and also assigned to the assignee hereof, describes a 3-level QC light source that emits light at two wavelengths by either of two mechanisms: (1) by a pair of vertical electron transitions at different wavelengths in a single quantum well, or (2) by a diagonal electron transition at one wavelength from one well into an adjacent well followed by a vertical electron transition at a different wavelength from the latter well. This source, also described by C. Sirtori et al. in Optics Lett., Vol. 23, pp. 463 (1998), exhibited well-resolved luminescence peaks at wavelengths of 8 .mu.m and 10 .mu.m. However, the transitions were inefficient, and it was difficult to optimize both at the same time. Consequently, laser action was achieved on only one transition from the upper level to the middle level. This publication, as well as both of the aforementioned patent applications, are incorporated herein by reference.
Thus, a need remains for a QC light source which is capable of efficient, simultaneous emission at multiple wavelengths, and especially for a QC laser capable of such emission at two or more wavelengths.